Optical photons are excellent flying qubits for future long-distance quantum networks due to negligible decoherence at room temperature. To date, quantum photonic technologies have focused on processing the spatial, temporal and polarization degrees of freedom of light. However, frequency encoding of information has had a profound impact on classical telecommunications, creating mature low-loss fiber-based and integrated photonics hardware that can be exploited to address challenges of scalability in photonic quantum networks. In this dissertation, we use tools from nonlinear optics to realize coherent frequency domain processing of single photons. We use quantum frequency conversion via Bragg scattering four-wave mixing (BS-FWM) to manipulate the spectral and temporal properties of single photons. We use an implementation of BS-FWM that achieves close to unity efficiency and ultra-low noise to develop a powerful toolbox that combines advantages of frequency encoding, fiber and integrated photonic technologies and nonlinear optics for scaling future quantum networks. The first application discussed in this thesis is a frequency-multiplexed single-photon source. Deterministic, high-quality sources of single photons are a crucial requirement for scalable photonic quantum information processing (QIP). The most widely used single-photon sources are based on nonlinear parametric processes that are inherently probabilistic. Active feed-forward switching and multiplexing of such probabilistically generated photons can be used to generate photons on demand if a sufficiently large number of modes are multiplexed. Schemes based on spatial and temporal multiplexing however suffer from prohibitive switching losses that significantly limit their performance. We implemented an alternative scheme based on frequency multiplexing that breaks this limitation. We used BS-FWM as a ‘frequency switch’ to multiplex frequency modes of a broadband probabilistic single-photon source. We demonstrated a 220% enhancement in single-photon generation rate while maintaining low noise properties ($g(2)$ = 0.07) essential for quantum applications. This approach has a unique potential to create a deterministic source of single photons on a chip-based integrated photonics platform. The next application we discuss is Hong-Ou-Mandel (HOM) interference with photons of distinct colors. In this work, we combine frequency-entangled photons generated on-chip together with Bragg-scattering four-wave mixing (BS-FWM) in fiber to demonstrate frequency-domain HOM interference with 95% visibility. BS-FWM coherently couples distinct frequency modes while preserving all quantum properties of the input fields and can therefore be used to create an active, tunable ‘frequency beam splitter (FBS)’. We observe a rich two-photon interference pattern including quantum beating, previously observed with cold-atomic systems. Remarkably, we observe high fidelity interference even though the photons propagate for much longer than their mutual coherence time, confirming that this is truly a two-photon interference phenomenon. In addition to fundamental novelty, this work establishes four-wave mixing as a tool for selective, high-fidelity two-photon operations in the frequency domain, which combined with integrated single photon sources provides a building block for frequency multiplexed photonic quantum networks. This demonstration will also enable applications such as frequency domain boson sampling, which we discuss in detail in this dissertation. Finally, we demonstrate a single-photon level time lens with picosecond resolution using BS-FWM. We discuss the conditions under which broadband phase-matching can be achieved with BS-FWM. A time lens draws on space-time duality and imparts a quadratic phase shift on the input signal. With this setup, we achieve a temporal magnification factor of 158 and resolve single-photon level pulses separated by 2.2 ps. Finally, we show that the temporal phase imparted by the BS-FWM pumps can be generalized to realize significantly more complex, unitary operations on broadband temporal modes. In particular, we use numerical optimization via steepest gradient descent to demonstrate temporal mode sorting of field orthogonal but intensity overlapping Hermite-Gaussian temporal modes. These results show that BS-FWM is a powerful tool for temporal mode quantum processing at the single-photon level.